1. Field of the Invention
The present invention relates to an exhaust gas-purifying catalyst and a method of manufacturing the same, and more particularly to an exhaust gas-purifying catalyst using a stabilized zirconia as a carrier or support, and a method of manufacturing the same.
2. Description of the Related Art
In recent years, the regulation of exhaust gas of motor vehicles is being made stricter, increasing the necessity for further decreasing the amounts of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOX) in the exhaust gases.
Hitherto, supported catalysts in which a noble metals (catalyst active component) such as rhodium is supported on a porous support have been widely used as catalysts for purifying exhaust gas from motor vehicles. Such supported catalysts are called three-way catalysts, because they can oxidize CO and HC and reduce NOX in the exhaust gas simultaneously.
Conventionally, γ-alumina is used as the support. However, γ-alumina undergoes phase-transition into α-alumina during use at high temperatures for a long period of time, and consequently the specific surface area of γ-alumina decreases. In addition, rhodium becomes solid-solutioned into the alumina during use at high temperatures, and the catalyst performances are lowered (see Japanese Patent Application Disclosure (KOKAI) No. 2001-347167).
Under the circumstances, support materials superior in thermal stability are required, instead of alumina. For example, Japanese Patent Application Disclosure (KOKAI) No. 2000-15101 discloses supporting rhodium on a zirconia support stabilized with an alkaline earth metal. Further, Japanese Patent Application Disclosure (KOKAI) No. 2002-518171 discloses a catalyst in which rhodium is supported on a zirconia support stabilized with a rare earth element.
Indeed, the stabilized zirconia supports are superior in thermal stability. However, it has been found by the present inventors that catalysts wherein a noble metal is supported on the stabilized zirconia support largely decrease in catalytic performances due to the fact that the surface noble metal particles aggregate or grow under the fluctuation of the atmosphere (alternate fluctuation between lean (oxidizing) atmosphere and rich (reducing) atmosphere during use at high temperatures, though they are superior in initial performances.